In a variety of applications, implantable medical devices are used for one or both of monitoring or delivering therapy to a patient. For example, cardiac pacemakers typically monitor electrical signals from the heart, i.e., an electrocardiogram (ECG), and deliver electrical stimulation to the heart, via electrodes. The electrodes may be located within the heart, and coupled to the pacemaker by intravenous leads, or may be positioned subcutaneously using any non-intravenous location, such as below the muscle layer or within the thoracic cavity, for example.
In the case of demand pacing, for example, a cardiac pacemaker monitors the ECG to determine whether an intrinsic cardiac depolarization, e.g., a P-wave or R-wave, occurs within a rate interval. If an intrinsic depolarization occurs, the pacemaker resets a timer and continues to monitor the electrical signals from the heart. If an intrinsic depolarization does not occur, the pacemaker delivers one or more electrical pulses to the heart, and resets the timer.
Many pacemakers have used analog circuitry to process the ECG, e.g., to detect P-waves and R-waves. Implementation of digital signal processing for this purpose would be desirable, but would require relatively high resolution analog-to-digital conversion of the ECG. Increased resolution for analog-to-digital conversion generally requires higher oversampling of the analog signal, or more complex comparator circuitry, both of which increase the amount of current drain associated with the analog-to-digital conversion. Increased current drain is a concern in implantable medical devices, and particular in primary cell devices, where it may shorten the life of the power source of the implantable medical device, thereby requiring earlier explantation and replacement of the implantable medical device. Minimization of power consumption is also desirable for implantable medical devices with rechargeable power sources to, for example, reduce the frequency of recharging events and thereby increase the convenience of the implantable medical device from the perspective of the patient.
Another example application for digital signal processing in implantable medical device is analysis of electrical signals within the brain, e.g., an electroencephalogram (EEG), sensed via electrodes. An implantable medical device may analyze an EEG to, for example, identify epileptic seizures, or other neurological issues. In some cases an implantable medical device may deliver electrical stimulation to the brain, or other tissue within the patient, in response to or based on the analysis of the EEG. Furthermore, digital signal processing may be used in implantable medical devices to analyze any of a variety of signals generated by any of a variety of sensors based on physiological parameters of a patient, such as pressure, impedance, temperature, or physical motion.